1. Field of the Invention
The present invention relates to a system for controlling the mass flow rate of vapor in a carrier gas.
2. Description of the Prior Art
In certain vapor-phase chemical processes, such as are encountered in the manufacture of semi-conductors, it is necessary to precisely meter the input mass flow rate of liquid chemicals as minor vapor-phase constituents of a flowing gaseous atmosphere.
In a typical system for the manufacture of semi-conductors a carrier gas such as hydrogen is introduced into a vaporizer where it is passed over or through a liquid source such as silicon tetrachloride to pick up vapor for discharge into a reaction chamber in which a portion of the vapor material is deposited upon a semi-conductor wafer or the like. The mass flow rate of the silicon tetrachloride vapor must be very accurately controlled in order to deposit a precise thickness of the vapor material on the wafer.
Regulation of vapor mass flow rate is accomplished in the prior art in various ways. In one system the temperature and pressure of the liquid in the vaporizer, and the flow of carrier gas through the vaporizer, are closely regulated to maintain constant the amount of vapor taken up by the carrier gas. This system is difficult to control because of variations which are present in the degree of saturation of the vapor in the carrier gas.
Another prior art vapor feed regulation system measures the ratio of vapor to carrier gas as the mixture leaves the vaporizer. This is done by thermal conductivity analysis, the flow rate of carrier gas being controlled to yield the desired vapor feed rate, as determined by the analysis. This type of system is limited by the accuracy of the thermal conductivity analysis, and the vapor is typically undesirably exposed to contamination from the heated conductivity cells which characterize the analysis. Typical of this type of prior art system is that which is disclosed and claimed in U.S. Pat. No. 3,650,151, issued Mar. 21, 1972 for FLUID FLOW MEASURING SYSTEM.
Yet another system of the prior art involves metering the rate of flow of the liquid source to a flash vaporizer where it is vaporized in a carrier gas stream. This system does not work well for extremely low flow rates, and particularly where chemicals are involved which are corrosive. The metering is imprecise and subject to contamination under these conditions.